Water used for domestic, agricultural, and industrial use is increasingly becoming contaminated with objectionable substances. Their hazards include health risks, damage to water transmission systems and their components, damage to processing equipment, and fouling of products, crops, cropland, and industrial sites.
Because this contamination has become so severe and pervasive, it is not surprising that many efforts have been made toward remediation. Remediation in this context means removal of volatilizable compounds that are dissolved in liquid such as water to be treated, or that are dissolved in water used to extract the contaminants from soil.
There are numerous physical and chemical processes for this purpose, such as reverse osmosis and chemical treatment. However these are mostly useful for relatively small applications, and are very capital intensive in waterwork applications where the flow rate of water to be treated is very high. Further, these processes frequently involve very costly chemical reagents and high energy costs.
Other remediation pathways have been employed, especially for high rate of flow applications. Their objective is to transfer the contaminant from water into a gas, usually air. The contaminant is encouraged to leave its solution in the water and enter the gas phase as a gas, crossing the interface between the water and the gas phase as it does so. The gas with the contaminant in it is conveyed away, and the concentration of contaminant in the water is reduced.
The effectiveness of such systems is greatly impacted by the total amount of surface area of interface between the water and the air. Common expedients to increase this surface area are found in counterflow towers, in which water flows downwardly while a current of air flows upwardly, making contact with the surface of the water. Increasing the interface area is commonly accomplished by filling the tower with a packing such as rings or plates to spread out the water for contact by the current of air.
Such equipment tends to be large, costly, and excessively consumptive of energy. It is large because the tower must accommodate bulky packing to provide sufficient interface area. It is excessively consumptive of energy because air must be passed in large amounts over the interface surface in order to keep the concentration of contaminant in the air low enough to encourage passage of the contaminant from solution into the air.
Because remediation does not produce income (except to the suppliers of the equipment), its cost, the cost of the real estate area it occupies, and the energy it consumes are subject to close and reluctant scrutiny.
Another known remediation pathway is to spray the water in the form of droplets from the top of a tower. As the droplets fall they encounter an upward counterflow of air. The interface is now on the surface of drops instead of on a sheet of water. At least theoretically the total area on the drops can be much larger than the total area provided in packed towers of similarly sized installations.
Both of these well-known systems face irreducible limits on the total surface area of their interfaces. In towers there can be only so much packing while still allowing contiguous space for sufficient air to pass through them. In spray towers there is a physical limitation on the density and size of the droplets if they are to remain discrete and separated so as to be contactible by the air. Excessive reduction of droplet size soon renders the droplet flow subject to entrainment in the air stream.
The foregoing examples illustrate the irreducible lower limits on the size of the equipment, mainly because of the inherent requirement for space to accommodate a given amount of interface in one arrangement and to prove space between droplets. for air flow that does not entrain the drops in the other. In these arrangements the situation is either not improved or is worsened if the system is operated at a sub-atmospheric pressure.
Henry's law indicates that the solubility of a volatile compound in water decreases along with a decrease in system pressure. It follows that transfer from water across an interface into air is favored by a reduction in pressure. While the rate of transfer across the interface between the water and the air will be the same for the interface in any system at the same pressure with identical concentrations in the water and in the air, the above physical constraints are ultimate limitations on the unit performance of the equipment at any pressure and temperature.
It is an object of this invention to overcome these limitations and to provide a process and process equipment which cost less to purchase and operate, while producing improved removal of the contaminant in equipment of considerably reduced size and footprint. Further, it is adaptable to a wider range of flow rates and operating pressures.
For example, a conventional counterflow tower which utilizes a downwardly flowing spray of water may require a 13 feet diameter tower, 19 feet high, and about 25 hp for its operation. As an example of its effectiveness, about 85% of tetrachloroethylene (TCE) or carbon dioxide present in the water will be removed. Their Henry's constants are about equal. The cost of such an installation tends to be about $210,000.00.
In contrast, an installation according to this invention requires a height of only about 7 feet for a separation chamber about 12 inches in diameter for the same flow rate of water, consuming only about 8 hp. This system will remove about the same amount of the same contaminant. The cost of this installation tends to be about $65,000.00. It is much smaller and less expensive to operate.
The superiority of results and costs of the installation and operation are evident and surprising.